The invention relates to a semiconductor electro-photonic device, and more particularly to a vertical-to-surface transmission electro-photonic device.
The stage of revolutions in the semiconductor material technology is entering into the age of developments in electro-photonic semiconductor devices including electronic and photonic devices for optical data transmission and information processing. The value and importance in realizations of high density and high speed parallel optical transmission, processing and computing application will be on the increase in the semiconductor industry. The optical transmission devices such as light emitting devices for such the optical transmission and processing may be divided into two types. One is an edge transmission device and the other is the vertical to surface transmission device. The vertical-to-surface transmission electro-photonic device will receive a great deal of attention increasingly as being suitable for a two-dimensional integration which is able to permit the high density and high speed parallel optical transmission, processing and computing application. Such the vertical-to-surface transmission electro-photonic device as integrated in two-dimensional arrays is required to possess excellent properties such as a high efficiency in the electronic-optical power conversion and a low electrical resistance for the above high density and high speed parallel optical transmission, processing and computing application. For the properties of the vertical-to-surface transmission electro-photonic device, failure of either the high efficiency in the electronic-optical power conversion or the low electrical resistance makes it difficult to accomplish the high density and high speed parallel optical transmission, processing and computing application. For that reason, it seems essential that the vertical-to-surface transmission electro-photonic device is required to possess the above both properties or the high efficiency in the electronic-optical power conversion and the low electrical resistance. So far as the inventor's knowledge, any of the conventional vertical-to-surface transmission electro-photonic devices have, however, possessed insufficient properties both in the electronic-optical power conversion efficiency and in the device resistance.
A typical structure of the conventional vertical-to-surface transmission electro-photonic device with a vertical cavity includes top and bottom reflective mirrors which sandwich intermediate layers. The intermediate layers constitutes a double hetero structure which comprises an active layer with a smaller refractive index and cladding layers with a larger refractive index sandwiching the active layer. Such the double hetero structure with a compositional variation provides a potential well to confine injection carriers and a refractive index discontinuity. The carrier confinement greatly enhances the utilization of injection carriers. The optical confinement causes the stimulated emission. The light or laser emission requires an injection of carriers or electrons and holes into the active layer. The injection of the carriers into the active layer is able to cause the population inversion of of electrons in the active layer. The population inversion of electrons causes the recombination of electrons and holes which causes the spontaneous emission of photons or light. The light caused by the spontaneous emission is confined in the active layer with the smaller refractive index sandwiched by the cladding layers with the larger reflactive index. The confinement of the light caused by the spontaneous emission causes the stimulated downward transition of electrons which is able to emits photons or light. In the surface emitting layer, the propagation of the light emitted by the stimulated emission appears in a vertical direction to a surface of the active layer. As described the above, since the light emitted from the active layer in the vertical direction is generated in the active layer by the spontaneous emission and the stimulated emission, an amount of the power of the light emitted from the active layer is defined by not only a magnitude of the carrier injection into the active layer but also a size of the active region in the vertical direction, namely a thickness of the active layer. Needless to say, a large thickness of the active layer is able to permit of a generation of a large power light emission in the direction vertical to surface of the active layer. Generally, however, the active layer is required to be extremely thin as comprising a potential well structure such as a single or multiple quantum well structure.
A further enhancement of the stimulated downward transition of electrons or the stimulated emission requires a further optical confinement for much more enhancement of the utilization of the light emitted from the active layer. The further optical confinement of the light emitted in the direction vertical to the surface of the active layer is made in the vertical cavity. The vertical cavity comprises a pair of reflective mirrors such as distributed Bragg reflective mirrors for reflecting the emitted light in the direction vertical to the surface of the active layer. The reflective mirrors are provided to sandwich the intermediate layers having the potential well structure formed by the active layer sandwiched by the cladding layers. The reflective mirrors are required to have a large reflectivity. The large reflectivity is provided by a large refractive index discontinuity due to the compositional variation which appears on an interface between a small refractive index semiconductor layer and a large refractive index semiconductor layer. For example, to obtain a large reflectivity, each of the reflective mirrors may comprise alternate laminations of large refractive index semiconductor layers such as AlAs layers and small refractive index semiconductor layers such as GaAs layers.
The large discontinuity of the reflactive index due to the compositional variation simultaneously provides a large energy band gap discontinuity. Then, the interface between the small refractive index semiconductor layer and the large refractive index semiconductor layer necessarily has a large energy band gap discontinuity in the direction across the interface. The large energy band gap discontinuity necessarily provides a large potential barrier to carriers or electrons and holes across the interface between the small and large refractive index semiconductor layers, for example, the interface between the GaAs/AlAs layers. In the conventional surface emitting layer device with the vertical cavity, the carriers are generally injected through the reflective mirror and the cladding layer into the active layer. In the conventional surface emitting laser device, a current pass of the injection carrier exists across the interface between the small and large refractive index semiconductor layers such as the GaAs/AlAs layers in the reflective mirrors. Then, the current pass of the injection carriers exists across the potential barriers which appear on the interface between the small and large refractive index semiconductor layers.
From the above descriptions, the reflective mirror with a large reflectivity suitable for optical confinement has a large potential barrier due to the large energy band gap discontinuity. In such the reflective mirror, the injection carrier necessarily experiences a large potential barrier. This provides the enhancement of an electrical resistance of the current pass in the laser device. The enhancement of the electrical resistance causes problems with a requirement of a large electrical power of the current injection for obtaining the necessary light emission as well as a generation of a large heat due to the current of the injection carrier through the potential barrier. Either the requirement of the large electrical power of the injection current or the generation of the large heat constitutes a bar to realize the high density integration in the two dimensional arrays and the high speed performance of the laser device with a lower power consumption.
To settle the above problems, the vertical-to-surface transmission layer device is required to possess a high efficiency in the electronic-optical power conversion and a low electrical resistance for the above high density and high speed parallel optical transmission, processing and computing application. The realization of the high efficiency in the electronic-optical power conversion requires the high reflectivity of the reflective mirrors which forms the vertical cavity to confine the stimulated emission light for a further enhancement of the stimulated downward transition of electrons or the stimulated emission. On the other hand, the realization of the low electrical resistance requires a bypass of the currents of the injection carriers to avoid the interfaces of the small and large refractive index semiconductor layers having a large difference in the energy band gap so that the injection carriers are permitted to be free from any large potential barrier. The majority carriers of the p-type semiconductor are holes whose effective mass is larger than the effective mass of electrons serving as the carriers in the n-type semiconductor.
Particularly, the potential barrier to holes having the large effective mass rather than that of electrons is a serious problem as providing a large enhancement of the electrical resistance to the injection carriers. Namely, the potential barrier in the reflective mirror made of p-doped semiconductor layers provides such serious problem.
To settle the above issue, surface emitting laser devices with any mesa structure were proposed in which a majority of the carriers injected from the p-electrode flows on a bypass avoiding the potential barrier against holes caused by the energy band gap discontinuity in the reflective mirror made of the p-doped semiconductors as illustrated in FIGS. 4A and 4B. Such the mesa structure surface emitting laser devices were reported by Kurihara et al. in 1993 Japan J. Applied Physics Vol. 32. pp. 604-608 as well as in Extended Abstracts of the 1992 International Conference on Solid State Device and Materials, pp. 598-600.
That conventional device or a vertical-to-surface transmission electro-photonic device with a mesa structure will hereinafter be described in detail with reference back to FIG. 1. A substrate 1 for the vertical-to-surface transmission electro-photonic device with a vertical cavity is made of n-GaAs semiconductor compound. A bottom distributed Bragg reflector mirror 2 comprises n-GaAs layers and n-AlAs layers which are alternately laminated in which the lamination comprises nine periods of the n-GaAs layer and n-AlAs layers, each of which has a thickness corresponding to a quater of a medium wavelength. The bottom distributed Bragg reflector mirror 2 is formed on a top surface of the n-GaAs substrate 1, although only a top pair of the n-GaAs layer and the n-AlAs layer is partially formed except in an n-electrode area. The n-GaAs layers and n-AlAs layers have a relatively large difference in those refractive index to serve as the distributed Bragg reflective mirror. The n-GaAs layers and n-AlAs layers with the large difference in the refractive index have also a relatively large difference in the energy band gap. A bottom cladding layer 3 made of n-Al.sub.0.3 Ga.sub.0.7 As is formed on the bottom distributed Bragg reflector mirror 2. An active layer 4 is formed on a top surface of the bottom cladding layer 3 in which the active layer comprises an i-In.sub.0.2 Ga.sub.0.8 As layer which forms a single quantum well structure. A top cladding layer 5 made of p-Al.sub.0.3 Ga.sub.0.7 As is formed on a top surface of the active layer 4. A top distributed Bragg reflector mirror 6 comprises p-GaAs layers and p-AlAs layers which are alternately laminated in which the lamination comprises eleven periods of the p-GaAs layers and p-AlAs layers, each of which has the thickness corresponding to a quater of the medium wavelength. The top distributed Bragg reflector mirror 6 is formed on a predetermined area in a top surface of the top cladding layer 5 in which the predetermined area corresponds to a light emitting area in the device, notwithstanding only a bottom pair of the p-GaAs layer and the p-AlAs layer is formed on an entire top surface of the top cladding layer 5. This results in that the top distributed Bragg reflector mirror 6 has a mesa structure. The p-GaAs layers and p-AlAs layers have a relatively large difference in those refractive indexes to serve as the distributed Bragg reflective mirror. The p-GaAs layers and p-AlAs layers with the large difference in the refractive index have also a relatively large difference in the energy band gap.
As described above, the conventional vertical-to-surface transmission electro-photonic device has a vertical cavity which comprises the top and bottom distributed Bragg reflector mirrors 6 and 2 in which the reflective mirrors 6 and 2 sandwich intermediate layers comprising the active layer 4 and the top and bottom cladding layers 5 and 3. Moreover, the conventional vertical-to-surface transmission electro-photonic device has not only the above mesa structure of the top distributed Bragg reflector mirror 6 but also high resistive regions 12 comprising proton-implanted regions. As illustrated in FIG. 1, the high resistive regions 12 are partially formed except in the light emitting area in the intermediate layers, namely the active layer 5 and the top and bottom cladding layers 5 and 3. It is desired that the high resistive regions 12 are tapering off as illustrated in FIG. 1, although it is difficult to form so. A horizontal distance between the high resistive regions 12 is equal to a length of the light emitting area under the mesa structure of the top distributed Bragg reflector 6.
A p-electrode 14 is formed to cover the mesa structure of the top distributed Bragg reflector mirror 6 and its adjacent portions. An n-electrode 13 is formed on the n-electrode region without the intermediate layers in which the bottom distributed Bragg reflector mirror 2 is exposed.
The description will subsequently be directed to fabrication processes for the above conventional vertical-to-surface transmission electro-photonic device.
With reference to FIG. 2A, the n-GaAs substrate 1 is prepared and the n-GaAs layers and the n-AlAs layers are epitaxially and alternately grown by molecular beam epitaxy on the top surface of the n-GaAs substrate 1 until the nine periods of the alternations of the n-GaAs layers and the n-AlAs layers are formed to serve as the bottom distributed Bragg reflector mirror 2. The n-Al.sub.0.3 Ga.sub.0.7 As epitaxial layer serving as the cladding layer 3 is grown by molecular beam epitaxy on the top surface of the bottom distributed Bragg reflector mirror 2. The non-doped In.sub.0.2 Ga.sub.0.8 As epitaxial layer serving as the active layer 4 is grown on the top surface of the bottom cladding layer 3 by molecular beam epitaxy. The p-Al.sub.0.3 Ga.sub.0.7 As epitaxial layer serving as the top cladding layer 5 is grown on the active layer 4 by molecular beam epitaxy. The p-GaAs layers and the p-AlAs layers are epitaxially and alternately grown by molecular beam epitaxy on the top surface of the top p-doped cladding layer 5 until the eleven periods of the alternations of the p-GaAs layers and the p-AlAs layers are formed to serve as the p-doped top distributed Bragg reflector mirror 6 thereby a vertical-to-surface emitting laser substrate 7 is completed.
With reference to FIG. 2B, a photo-resist film is formed on a top surface of the p-doped top distributed Bragg reflector 6 in the vertical-to-surface emitting laser substrate 7. A patterning for the photo-resist film is accomplished so that the photo-resist film is partially removed and a photo-resist pattern remains only in the light emitting area in which the mesa structure of the p-doped top distributed Bragg reflector will be formed.
With reference to FIG. 2C, except for the bottom one or two periods of the p-GaAs layer and the p-AlAs layer, the p-doped top distributed Bragg reflector mirror 6 is selectively removed by a reactive ion-etching 9 using a chlorine gas and the photo-resist pattern 8 so that the mesa structure 10 of the p-doped top distributed Bragg reflector 6 is defined in the light emitting area covered by the photo-resist mask 8.
With reference to FIG. 2D, an ion-implantation of proton in a vertical direction is accomplished by use of the photo-resist mask 8 so that proton is implanted into the epitaxial intermediate layers except in the light emitting area covered by the photo-resist mask 8. This results in that the epitaxial intermediate layers except in the light emitting area covered by the photo-resist mask 8 become high resistive regions 12 of the proton implanted regions. So far as the proton or other impurity implantations in the vertical direction are concerned, the light emitting area is defined by the photo-resist mask 8 used for the proton implantation. In the above processes, the photo-resist mask 8 was used not only in the dry etching process to form the mesa structure 10 of the p-doped top distributed Bragg reflector mirror 6 but also in the proton implantation process to form the high resistive regions 12. For those reasons, the size of the photo-resist mask 8 is able to define not only a horizontal size of the mesa structure 10 of the p-doped top distributed Bragg reflector mirror 6 but also a size of the light emitting area surround by the proton implanted regions as the high resistive regions 12.
With reference to FIGS. 2E and 2F, the photo-resist mask 8 is removed, after that a photo-resist film is formed on an entire surface of the device to cover the mesa structure 10 of the p-doped top distributed Bragg reflector mirror 6 and peripheral flat surface of the device. The photo-resist film is patterned so that part of the photo-resist film only in the n-electrode region is removed to serve as a photo-resist mask. A selective wet etching is accomplished by use of the photo-resist mask so that the bottom pair of the p-GaAs/AlAs layers involved in the p-doped top distributed Bragg reflector mirror 6 is partially etched but only in the n-electrode region. Subsequently, the p-doped top cladding layer 5, the active layer 4 and the bottom cladding layer 3 are partially etched in turn in the n-electrode region. Further, a top pair of the n-GaAs/AlAs layers involved in the n-doped bottom distributed Bragg reflector 2 is partially etched in the n-electrode region. The above wet etching is accomplished until at least the n-doped bottom distributed Bragg reflector mirror 2 is exposed.
With reference to FIG. 2G, an n-electrode is formed on the exposed surface in the n-electrode region of the n-doped bottom distributed Bragg reflector mirror 2. A p-electrode is formed to cover both the mesa structure 10 of the p-doped top distributed Bragg reflector mirror as well as flat portions adjacent to the mesa structure 10 of the p-GaAl/AlAs layers involved in the p-doped top distributed Bragg reflector mirror 6 thereby the fabrication processes for the conventional vertical-to-surface transmission electro-photonic device is completed.
As to the ion-implantation of proton in the vertical direction to form the proton implanted regions as the high resistive region, ideally, the proton is implanted at a predetermined energy such that a proton concentration profile in the vertical direction has a peak value in the active layer 4 thereby a cross sectional definition of the proton implanted region is tapering off toward the light emitting area. So far as the proton implantation or other impurity implantation in the vertical direction is concerned, it is, however, difficult to obtain the above tapering cross sectional definition of the proton implanted region. Actually, somewhat of the implanted protons is subject to remaining at portions adjacent to the surface exposed to the proton implantation around the mesa structure 10 of the p-doped top distributed Bragg reflector mirror 6. For that reason, in the area adjacent to the mesa structure 10, the bottom pair in the p-doped top distributed Bragg reflector mirror 6 and the p-doped top cladding layer 5 become resistive regions.
In the p-side of the device, it is important to suppress the carrier to show a downward current through the mesa structure 10 including the alternating laminations of the GaAs/AlAs layers which forms relatively large potential barriers due to relatively large energy band gap discontinuities. The large energy band gap discontinuity is caused by the compositional discontinuity of the GaA/AlAs layers. The compositional discontinuity is necessary to permit the large reflectivity possessed by the p-type top distributed Bragg reflector of the mesa structure 10.
As described above, it could be understood that the mesa structure 10 of the p-type top distributed Bragg reflector has a large electrical resistance to the majority carriers or holes. For that reason, a large part of the carriers or holes is supplied from a peripheral portion of the p-electrode 14 around the mesa structure 10 and subsequently flow through the top pair of the p-GaAs/AlAs semiconductor layers and the p-type top cladding layer 5 and then injected into the active layer 4. At this time, the transmitted carriers are subjected to the horizontal carrier confinement into the light emitting area by the high resistive regions 12 of the proton implanted regions with the tapering structure.
The conventional device is, however, engaged with the following problems. As described above, a large part of the injection carriers flows from the peripheral portion of the mesa structure 10 toward the light emitting area in the active layer 4 and receives the carrier confinement by the high resistive regions 12 of the proton implanted regions. Namely, a large part of the injection carriers flows part of the bottom pair of the p-GaAs/AlAs and the p-cladding layer 5 over the tapering portions of the high resistive regions 12 around the mesa structure 10. However, as described above, the actual proton implantation in the vertical direction necessarily permits somewhat of the protons not to reach the active layer 4 and thus to remain in the part of the p-GaAs/AlAs layers and the p-cladding layer 5 over the tapering portions of the high resistive regions 12 around the mesa structure 10.
From the above, it could readily be appreciated that the part of the p-GaAs/AlAs layers and the p-cladding layer 5 over the tapering portions of the high resistive regions 12 is a somewhat high resistive region not so much as the high resistive regions 12. The majority of the carriers or holes flows through the somewhat resistive part the p-GaAs/AlAs layers and the p-cladding layer 5 over the tapering portions of the high resistive regions 12. This provides the effective and actual increase of the electrical resistance of the device. This makes it impossible to provide a low electrical resistance which is one of the most important factors for the vertical-to-surface light emitting device.
To settle the above serious problem, an alternative proposal for the vertical-to-surface light emitting device have been known in the art, to which the present invention pertains. FIG. 3 illustrates a cross sectional structure of the another conventional vertical-to-surface light emitting device with the vertical cavity. It could readily be appreciated that the another conventional vertical-to-surface light emitting device as illustrated in FIG. 3 has a remarkable difference in the structure from the conventional vertical-to-surface light emitting device as illustrated in FIG. 1. The remarkable difference appears in a horizontal size of the active layer 4 defined by the tapering portions of the high resistive regions 12 of the proton implanted regions or in a horizontal distance between the high resistive regions 12 of the proton implanted regions. While the conventional vertical-to-surface light emitting laser as illustrated in FIG. 1 has the same horizontal distance between the high resistive regions 12 as the width of the mesa structure, the another conventional vertical-to-surface light emitting laser as illustrated in FIG. 3 has a larger horizontal distance D2 between the high resistive regions 12 than the width D1 of the mesa structure 10. Such a larger horizontal distance D2 between the high resistive regions 12 than the width D1 of the mesa structure 10 is able to permit the device to be free from the above problem as to the high electrical resistance. In the alternative conventional vertical-to-surface light emitting device, the majority of the carriers or holes supplied from the flat and peripheral part of the p-electrode 14 around the mesa structure 10 is able to flow through proton free p-type epitaxial layers into the active layer. Almost no carriers or holes flows through the somewhat resistive part of the p-type epitaxial layers over the tapering portions of the proton implanted high resistive regions 12 into the active layer 4.
Such the vertical-to-surface light emitting device with the larger carrier injection area of the active layer 4 illustrated in FIG. 3 is, however, engaged with the following disadvantage. The injection carriers receive almost no carrier lateral confinement by the proton implanted high resistive regions 12. This results in a low current density of the injection carriers or holes into the active layer 4 in the enlarged carrier injection area with the wide width of D2. The low current density of the injection carriers into the enlarged carrier injection area of the active layer 4 results in a reduction of the power of the light or laser emitted from the active layer. A strong light or laser emission requires a further large injection carrier which leads to a reduction in the electric-optical conversion efficiency. Such vertical-to-surface light emitting device including the enlarged carrier injection area of the active layer is unavoidably engaged with the disadvantage in the low efficiency in the electronic-optical conversion.
It has therefore been required to develop a novel vertical-to-surface transmission electro-photonic device which possesses not only a high efficiency in the electronic-optical conversion but also an extremely low electrical resistance.